A particle of mass m and charge q moves with a constant velocity v along the positive `x` direction. It enters a region containing a uniform magnetic field B directed along the negative z direction, extending from `x=a` to `x=b`. The minimum value of v required so that the particle can just enter the region `xgtb` is

Options

A. `( q b B)/( m)`
B. `( q ( b-a)B)/( m ) `
C. `( qaB)/( m ) `
D. `( q( b+a)B)/(2m)`